Mono-olefins with two to four carbon atoms per molecule can be converted at a temperature of 325.degree.-700.degree. C. into aromatic hydrocarbon mixtures by contacting the olefins with a catalyst containing a crystalline aluminum silicate with a special structure. Such crystalline aluminum silicates are characterized by the fact that after calcination in air for one hour at 500.degree. C. they possess the following distinguishing features:
(a) an X-ray powder diffraction pattern containing as strongest lines the four lines listed in Table A, and
TABLE A ______________________________________ d(.ANG.) ______________________________________ 11.1 .+-. 0.1 10.0 .+-. 0.1 3.84 .+-. 0.07 3.72 .+-. 0.06 ______________________________________
(b) when the composition of the silicate is expressed in moles of the oxides, the SiO.sub.2 /Al.sub.2 O.sub.3 mol. ratio is 25-400.
The above mentioned process results in a product in which, besides the desired C.sub.5.sup.+ hydrocarbons, C.sub.4.sup.- hydrocarbons formed as byproducts also occur. The C.sub.5.sup.+ fraction present in the product is more valuable according as it has a higher aromatic content.
The aromatic content of the C.sub.5.sup.+ fraction obtained depends to a large extent on the chosen reaction temperature and SiO.sub.2 /Al.sub.2 O.sub.3 mol. ratio of the crystalline silicate. According as a higher reaction temperature and/or a crystalline silicate with a lower SiO.sub.2 /Al.sub.2 O.sub.3 mol. ratio is used in the process, a product is obtained whose C.sub.5.sup.+ fraction has a higher aromatic content. For both these measures, however, the preparation of a product whose C.sub.5.sup.+ fraction has a higher aromatic content is accompanied by a reduction in the yield of the C.sub.5.sup.+ fraction. A particular relationship exists between the increase in the aromatic content of the C.sub.5.sup.+ fraction and the reduction in the yield of the C.sub.5.sup.+ fraction, so that, in principle, for every aromatic content of the C.sub.5.sup.+ fraction being prepared, there is a particular potential yield of the C.sub.5.sup.+ fraction. This relationship is unfavorable to the extent that only low yields of C.sub.5.sup.+ fractions with a relatively high aromatic content can be obtained. Since a possible application of the process on a commercial scale depends not only on the aromatic content of the C.sub.5.sup.+ fraction, but also on the potential yield of that C.sub.5.sup.+ fraction, there is a practical limit to the maximum aromatic content of the C.sub.5.sup.+ fraction being prepared, due to the fact that the corresponding yield of the C.sub.5.sup.+ fraction must still be acceptable.
It has recently been found that considerably better results can be obtained in the above-mentioned process if, as crystalline silicate with the previously mentioned special structure, a silicate containing gallium instead of aluminum is used, and if, moreover, the following two requirements are met:
(a) when the composition of the gallium silicate is expressed in moles of the oxides, the SiO.sub.2 /Ga.sub.2 O.sub.3 mol. ratio should be 25-250, and
(b) if the gallium silicate has a SiO.sub.2 /Ga.sub.2 O.sub.3 mol. ratio of 100-250, the catalyst should be subjected one or more times to a two-stage treatment, hereinafter termed "redox treatment" for short, comprising a first stage in which the catalyst is contacted for at least 15 minutes at a temperature of 350.degree.-700.degree. C. with a hydrogen-containing reducing gas, followed by a second stage in which the catalyst is contacted for at least 15 minutes at a temperature of 350.degree.-700.degree. C. with an oxygen-containing oxidizing gas.
In the case of the present crystalline gallium silicates corresponding with the previously mentioned crystalline aluminum silicates, the higher the reaction temperature and/or the lower the SiO.sub.2 /Ga.sub.2 O.sub.3 mol. ratio of the silicate employed, the higher the aromatic content of the C.sub.5.sup.+ fraction of the product obtained, and also that for both measures the preparation of a product whose C.sub.5.sup.+ fraction has a higher aromatic content is accompanied by a reduction in the yield of the C.sub.5.sup.+ fraction. For the crystalline gallium silicates, there is also a relationship between the increase in the aromatic content of the C.sub.5.sup.+ fraction and the reduction in the yield of the C.sub.5.sup.+ fraction, whereby, in principle, for every aromatic content of the C.sub.5.sup.+ fraction being prepared there is a particular potential yield of that C.sub.5.sup.+ fraction. The important difference between the crystalline aluminum silicates and the present crystalline gallium silicates is that the previously mentioned relationship for the gallium silicates is considerably more favorable, so that considerably higher yields of C.sub.5.sup.+ fractions with a relatively high aromatic content can be obtained than is the case for the aluminum silicates.
It has now been found that further improvement of the relationship between the yield of C.sub.5.sup.+ fraction and the aromatic content thereof can be achieved by carrying out the conversion in two stages. First of all, the feed is contacted in a first stage at a temperature of 325.degree.-550.degree. C. with a catalyst containing a crystalline metal silicate with the previously mentioned special structure, which silicate is further characterized in that in the formula representing the composition of the silicate expressed in moles of the oxides and wherein, apart from SiO.sub.2, one or more oxides of a trivalent metal selected from aluminum, iron, gallium and boron are present, the SiO.sub.2 /X.sub.2 O.sub.3 mol. ratio amounts to 25-400. Subsequently, the reaction product from the first stage is separated into a C.sub.4.sup.- and a C.sub.5.sup.+ fraction, and the C.sub.4.sup.- fraction is contacted in a second stage at a temperature of 450.degree.-700.degree. C. that is at least 50.degree. C. higher than the temperature employed in the first stage with the previously mentioned catalyst, defined in the description of the improved single-stage process, that contains a crystalline gallium silicate. Finally, the reaction product of the second stage is separated into a C.sub.4.sup.- and C.sub.5.sup.+ fraction and the separated C.sub.5.sup.+ fractions are mixed. Comparison of the results obtained during the preparation of aromatic hydrocarbon mixtures on the basis of a given feed and with use of either the now proposed two-stage process or the previously described improved single-stage process, employing the defined catalyst containing a crystalline gallium silicate has revealed that if the processes were aimed at the preparation of a C.sub.5.sup.+ fraction with a particular aromatic content, the two-stage process gave a higher yield of C.sub.5.sup.+ fractions, whereas if the processes were aimed at a given yield of C.sub.5.sup.+ fraction, the two-stage process produced a C.sub.5.sup. + fraction with a higher aromatic content. In the case of the two-stage process now proposed, it is essential that the C.sub.4.sup.- fraction of the product from the first stage is used as feed for the second stage, as it has been found that if the C.sub.5.sup.+ fraction of the product from the first stage or the total reaction product of the first stage is used as feed for the second stage instead of the C.sub.4.sup.- fraction of the product of the first stage, a result is achieved which is even less favorable than that obtained in the improved single-stage process.